You say "filthy" like it's a bad thing! ;-) I prefer Snape when he is snarky and not inviting Harry to live with him, though, and I couldn't quite picture Snape saying some of the later dialogue in the story. He's sexiest when he's gruff.

One note on the flow of the text: During the part of the story where concurrent action is happening (Harry is faking sick and going to the dormitory to change; Snape is faking sick and sending his students away) there is a lack of notation there to let the reader know both things are happening at the same time. I had to read it a few times to understand what was going on. Consider adding a few extra paragraph breaks, asterisks or even the combining two paragraphs and separating with a word like "Meanwhile..." (or perhaps something that sounds less like a Superman comic, if you prefer.) :-)

Hey! This story was very well written and dang...the sexual scene was HOT! HOT! HOT! Hope you have more stories in the near future because I know I'll be making sure to stop by and read them. Man, I just love a Sev/Har pairing cause I think there perfect for one another. Well, cool fic and keep it up! (-)